1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More specifically, the present invention is in the field of power management circuits and systems.
2. Background Art
The storage and on-demand delivery of electrical energy is becoming of increasing importance as the shift from fossil fuel based technologies to green technologies gains momentum. Gas/electric hybrid automobiles, for example, typically utilize arrays of secondary batteries that are alternately charged and discharged in response to vehicular operation. Those secondary battery packs may constitute a substantial portion of the cost of a gas/electric hybrid vehicle, and their performance, and in particular their longevity, may significantly influence consumer willingness to invest in the initially costlier vehicle purchase price.
Typical energy storage packs are assembled from individual batteries or energy cells and each unit is often assumed or selected to be nominally identical. In practice, the batteries or other energy cells will have individual performance parameters, such as storage capacity and/or resistance, that vary somewhat from unit to unit. The distribution or variation among cells may arise, for example, from process variation at the time of manufacturing, from unequal wear during use cycles, and other non-use related degradation of the energy cells. In general, the distribution of the variations is often seen to grow wider as the energy cells grow older. Unfortunately, moreover, the longevity and capacity of a group of energy cells used collectively, such as a battery pack, is typically determined by the weakest cell or battery in the group.
When a collection of cells is used in the discharge process, the first cell that reaches the full discharge point (e.g., the weakest cell) stops the discharge process despite the strong cells perhaps having remaining charge that is unused. When such a collection of cells is charged, the first cell that reaches the full charge point (e.g., the weakest cell) stops the process while the stronger cells remain undercharged. Over repeated cycles, the balance among cells may become worse. Furthermore, the capacity of the cell will degrade with each cycle, and, depending on the depth of discharge, the cells will typically age at different rates when discharged at different depth of charge. This implies that the weakest cell will typically experience the greatest depth of discharge which will age the cell faster making it a still weaker cell.
Conventional approaches to providing management of battery performance tend to focus on balancing one aspect of the cells at a given fixed time (usually when the pack is not in use). At that one point in time, the stronger cells are bled down to the level set by the weakest cell. For example, after a charge cycle, all of the voltages for each cell are different, representing a variation in state of charge. In other active implementations, charge is transferred from one cell to another (e.g., either to adjacent cells or on a random basis) until the voltage is balanced. The battery management system (BMS) bleeds charge until the all cells achieve the same voltage as the weakest cell. During normal operation (e.g., discharge and/or charge), the cells are typically discharged and/or charged at the same current. Because the charge capacity typically varies from cell to cell, the energy of each cells is drawn down or charged up at a different rate and the end of each cycle is limited by the weakest cell.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing a power management solution capable of maintaining charge balance during dynamic operation which allows cells to have improved performance and increases the longevity of an energy cell array as a whole.